This invention relates to focus detecting devices, and more particularly to a focus detecting device which is applicable to cameras and other optical instruments.
Focus detection methods have been proposed in the art, in which the focus is detected by utilizing the fact that the contrast, spatial frequency or light quantity of an object is changed according to the degree of focussing. In a first one of the conventional methods, a photoelectric element such as a CdS cell having a particular contrast sensing characteristic is utilized. In a second method, a photoelectric output signal obtained by mechanically vibrating or rotating an optical member is utilized. In a third method, similar to a range/view finder, two outputs of light detecting members disposed at different positions are utilized for detecting the focus.
However, the first method suffers from a serious drawback that it is difficult to detect the focus with high accuracy because it is difficult to manufacture photoelectric elements having the same light response characteristic. Also, the output variation rate of the photoelectric elements is low before and after the focussing is obtained in detecting the focus under conditions of low intensity of illumination. The second method is disadvantageous in that it is difficult to miniaturize the camera because it has mechanically movable components, and it consumes large amounts of electric power. The third method is also disadvantageous in that it is difficult to change the lens with that type of system.